1. Field of the Invention
The present invention relates to a driving method for a passive addressing type liquid crystal display device.
2. Discussion of the Background
As a basic driving method for a passive (multiplexed) addressing type liquid crystal display element, there has been proposed a line successive selection method (for instance, APT: Alt Pleshko Technique) or IAPT (Improved Alt Pleshko Technique as an improvement of APT). This technique is very useful as a multiplex driving method since ON-OFF levels can be easily driven. However, since the direct addressing type liquid crystal display device does not use active elements such as TFTs, there was a problem of reduction of contrast ratio due to frame response when a liquid crystal display element of fast response was used.
In order to solve such problem, a multiple line selection method has been proposed whereby it has been possible to display a picture having a high contrast ratio at a high speed. Further, in order to achieve the same purpose as described above, an attempt of using a whole line simultaneous selection method (AA: Active addressing) has been reported. Thus, a new addressing technique has been developed with the result of improving a quality of display.
There has been an increased demand for displaying pictures with many gradation levels for personal computers, TVs etc. and liquid crystal display devices as well. Several methods have been used for displays with gradation. In an active type driving method using transistors, diodes, or the like, an amplitude modulation can be easily achieved by using voltage pulses whose pulse height is varied depending on gradation levels of data to be displayed. This is because voltages applied to liquid crystal are basically of a static waveform.
In a passive multiplexed type driving method which typically uses a STN (super-twisted nematic) liquid crystal element and so on, however, there is a voltage change in a non-selection time when voltage pulses whose pulse height is varied depending on gradation levels of data to be displayed are simply applied to the element. Under the circumstances, there have been used or proposed several methods to display gradation levels in the passive multiplexed type driving method.
In the conventional driving methods of driving STN, there have been proposed and used a frame rate control method (FRC) and a pulse width modulation method (PWM) in order to obtain a display with gradation. Recently, an amplitude modulation method (AM) has been proposed. In the following, description will be made briefly on the proposed methods, and then, description will be made on problems caused when these methods are applied to the multiple line selection method.
(1) Frame rate control (FRC)
A gradation display is made with use of a plurality of frames. Namely, an intermediate tone is formed in response to the number of ON and OFF as a binary state. For instance, when three frames are used, four states, ON/ON/ON, ON/OFF/ON, OFF/ON/OFF and OFF/OFF/OFF can be displayed.
However, when a picture having many gradation levels is to be displayed with use of the FRC method, there may cause a flicker because an increased number of frames takes a long time to complete a display. Practically, the FRC method is combined with a spatial modulation method for shifting spatially phases to thereby avoid the occurrence of the flicker. However, the proposed method is considered to be difficult to obtain a picture having more than 16 gradation levels.
Another important problem in the FRC method resides in difficulty in applying it to a video display. For instance, in a display of dynamic picture, the display should be completed in a period in which a dynamic picture is changed. Accordingly, it is impossible to use many frames, and a display of many gradation levels is difficult.
For instance, when a frame frequency of 120 Hz (a generally used frequency, and the length of a frame is 8.3 ms) is used and a dynamic picture of 30 pictures per sec.(30 Hz) is to be displayed, it is necessary to complete the display in 4 frames. In this case, the number of gradation levels which can be displayed is only about 5 to 8. Thus, the FRC method was insufficient to display a dynamic picture having many gradation levels.
(2) Pulse width modulation (PWM)
In this method, a selection time period is divided into, for instance, a 2.sup.n number of sub-periods, and an ON state and an OFF state are distributed to the sub-periods. This method can be considered as such a technique that the FRC method is carried out in a frame. However, this method has a drawback that ununiformity becomes large in a display as the density and the gradation levels of a display is increased because the driving frequency is increased in proportion to the number of divided time periods.
(3) Amplitude modulation (AM)
As described before, it is impossible to multiplexed driving the passive addressing type LCD by simply applying voltage pulses whose pulse height is varied depending on gradation levels of data to be displayed, and it is necessary to avoid a change of the effective voltage to pixels in a non-selection time. For this purpose, there have been proposed two techniques: application of a plurality of voltages and use of an imaginary electrode.
In the former technique, different data (column) voltages are applied to two or more frames, or a selection time period is divided into two or more time periods wherein different data voltages are applied to the divided time periods. The application of a plurality of voltages makes the effective voltage in a non-selection time constant whereby a desired gradation display can be obtained. Specifically, the voltages corresponding to two kinds of data as shown in Formula 1 may be applied to each frame, or the two kinds of voltages may be applied by exchanging them in a selection time period.
Formula 1 EQU d+(1-d.sup.2).sup.0.5 EQU d-(1-d.sup.2).sup.0.5
where d indicates display data (ON: -1, OFF: 1)
Hereinbelow, the data shown in Formula 1 are referred to as divided data. The application of only part of the divided data does not render the effective voltage value to be a predetermined constant value, and therefore, addressing is not completed. Accordingly, in a case that the divided data are applied to each of the frames, the frames are referred to as subframes in order to distinguish them from the ordinary frames.
The divided data are featurized by including components which vary depending on gradation levels of data. However, since the divided data respectively include a correction term, (.+-.(1-d.sub.2).sup.0.5), the effective value of voltages applied to pixels in a non-selection time can be kept constant. New divided data can be produced on the basis of the respective divided data, whereby more than two kinds of divided data can be used.
In this technique, a device capable of supplying a plurality of voltage levels is required. In order to display K gradation levels, voltages of a (2K-2) number of levels are required. Namely, a display of 8 gradation levels requires 14 voltage levels. As the number of gradation levels increases, the number of voltage levels increases. An increased number of voltage levels will cause an increased manufacturing cost. Further, a state of display is basically determined by applying two voltage levels. Accordingly, if a time interval of applying a unit voltage (a width of pulses of a voltage) is made constant, the length of frames for completing a display is twice as in the conventional technique.
Another method of avoiding a change of the effective voltage values to non-selected pixels is to provide at least one line of imaginary electrode, wherein selection lines are driven so as to display data for the imaginary row electrode, or voltage levels which have been imaginary determined may be applied to the selection lines. This method had an advantage that there is no substantial change in frequency because the length of frame is not made double. However, this method has disadvantages that operations with all line data are necessary, and the number of voltage levels to be supplied is remarkably increased due to the sum of the number of gradation levels and the number of correction levels. In particular, the increase of the number of voltage levels is a serious problem which has prevented the spreading of the AM method. The above-mentioned two methods include a technique referred in U.S. Ser. No. 08/098,812 and a technique referred to as a pulse height modulation (PHM) disclosed in Japanese Unexamined Patent Publication No. 89082/1994 (or EP 569974).
As described above, the technique for displaying gradation with use of the amplitude modulation method inevitably caused a complicated circuit structure and the necessity of using drivers for a number of levels, which invited a substantial increase of manufacturing cost.
(4) Problems in multiple line selection method
In the multiple line selection method, the above-mentioned conventional driving method can be utilized with a certain modification. For instance, when a gradation display is conducted in accordance with the amplitude modulation method in addition to using a plurality of divided data, each of the divided data is displayed in accordance with the multiple line selection method whereby a gradation display is possible. Namely, column signals are formed by the orthogonal transformation of the divided data with use of a predetermined selection matrix (an orthogonal matrix).
However, the before-mentioned problem on the gradation display equally takes place in the multiple line selection method. The frequency rate control method (FRC) and the pulse width modulation method (PWM) have the same problem as the successive line selection method concerning the difficulty of obtaining a display having a number of gradation levels. In the amplitude modulation method, an increase of the maximum voltage value and an increased number of voltage levels due to selecting simultaneously a plurality of lines cause more serious problem in comparison with the successive line selection method. In other words, in the multiple line selection method, calculation with use of an orthogonal function is needed whereby a large number of voltage levels are necessary for display. Further, the construction of circuit is complicated. An increased number of gradation levels causes a big problem of pushing up manufacturing cost.
The multiple line selection method utilizing the amplitude modulation method requires a large number of voltage levels even though the number of gradation levels is small and the number of lines simultaneously selected is small. For instance, in a case that the number of gradation levels to be displayed by the AM method is only 8 and each line is successively selected, 12 voltage levels are needed because 6 gradation levels in 8 gradation levels are used for data of intermediate values, and it is necessary to provide voltage levels as twice as the number of data for the intermediate values even in a case that each line is successively selected. In the application of the AM method to the multiple line selection method wherein addition and subtraction of voltage levels are conducted at the time of the orthogonal transformation, the number of voltage levels is fairly increased even though the number of simultaneously selected lines is small. For instance, when L (the number of simultaneously selected lines)=3, voltage levels of about 8.sup.3 =512 are required. Namely, the amplitude modulation in the multiple line selection method requires column drivers of a very high degree of resolution (more than 8 bits, preferably 10-12 bits). If drivers having a smaller number of levels are used, there produces data error.